A glass bulb for cathode ray tube usually comprises a front panel on which an image is to be displayed, a funnel disposed behind the panel and sealed with the panel, on outside of which a deflection yoke is to be attached, and a neck welded with the funnel for accommodating an electron gun therein.
The panel on which an image is to be displayed has a generally rectangular shape, and also a wide open end of the funnel sealed with the panel has a generally rectangular shape similar to that of the panel. Further, the neck in which an electron gun is to be accommodated has a cylindrical shape, and also a narrow open end of the funnel welded with the neck has a similar circular shape.
In order to allow a cathode ray tube to display a proper image without displacement of color, it is important to assemble it while precisely positioning the respective axes of the panel, the funnel and the neck with respect to the tube axis of the glass bulb. In order to accomplish this, on outer walls of sides of the funnel which are adjacent to each other is provided a plurality of positioning reference portions which are to be used with being abutted to a jig at the time of sealing with the panel (See, for example, Japanese Unexamined Patent Publications JP-A 63-10447 (SHO-63, 1988) and JP-A 9-245646 (HEI-9, 1997)), and reference surfaces of the positioning reference portions are formed at a predetermined distance from the axis of the funnel.
FIG. 6 is a perspective view of a conventional funnel for cathode ray tube disclosed in the above-mentioned publications and the like, and FIG. 7 is a section view of essential part of a positioning reference portion thereof. In each drawing, the reference numeral 1 denotes a funnel, the reference numeral 2 denotes a wide open end, the reference numeral 3 denotes a narrow open end, the reference numeral 4 denotes a maximum profile line, the reference numerals 5a, 5b, 5c denote positioning reference portions, the reference numerals 6a, 6b, 6c denote reference surfaces of the respective reference portions, and the reference numerals 7a, 7b, 7c denote upper end edges of the respective reference portions.
Conventionally, these upper end edges 7a, 7b, 7c of the positioning reference portions of the funnel for cathode ray tube are formed at positions overlapping with the maximum profile line 4 formed by mold matching of a forming mold 10 and a shell mold 11.
In other words, as shown in FIG. 8, the aforementioned funnel 1 is generally press-molded by supplying a forming mold consisting of a bottom mold 10 and a shell mold 11 with a molten glass gob of high temperature, and then causing a plunger mold 12 to move down so as to extend the molten glass gob by pressing. After molding, the plunger mold 12 is moved up, and after removing the shell mold 11, the funnel 1 is removed from a bottom mold 10.
Molding surfaces for the positioning reference portions 5a, 5b, 5c are formed as recess portions in a bottom mold 10, and formed in the uppermost part of a bottom mold 10 in a state that upper portions thereof are free due to the fact that the funnel 1 will be removed from the bottom mold 10 after press molding. Consequently, the upper end edges 7a, 7b, 7c of the positioning reference portions are located at the positions that overlap with the maximum profile line 4 formed by mold matching of the bottom mold 10 and the shell mold 11.
As described above, the upper end edge 7a of the positioning reference portion 5a (ditto for 5b, 5c) of the conventional funnel overlap with the maximum profile line 4 formed in the vicinity of the wide open end 2 of the funnel 1 as a result of mold matching of forming mold consisting of the bottom mold 10 and the shell mold 11, and moreover, the positioning reference portion 5a is formed into a shape projecting to the outside from the maximum profile line 4.
For this reason, during press molding of the funnel, the positioning reference portion 5a is to be formed of glass of which temperature has slightly decreased in the substantially final stage of molding where the molten glass gob supplied to the bottom mold 10 is extended by pressing toward the neighborhood of the wide open end formed by the shell mold 11, and moreover the positioning reference portion 5a has a shape projecting to the outside from the outer wall of the funnel, with the result that not-filled (a defect that the positioning reference portion 5a is not completely filled with the glass) or wrinkling was likely to occur in the corresponding reference surface 6a. 
Furthermore, chipping sometimes occurs in the corner between the reference surface 6a of the positioning reference portion 5a and the upper end edge 7a because of flash of glass formed by mold matching of forming mold consisting of the shell mold 11 and the bottom mold 10. Additionally, in the positioning reference portion 5a of which wall thickness suddenly changes, small cracking caused by concentration of heat stress at the upper end edge 7a used to cause a breakage on the entire funnel.
It is therefore an object of the invention to suppress molding defects such as not-filled, wrinkling and chipping in a positioning reference portion formed on an outer wall of a funnel for cathode ray tube, and avoid occurrence of small cracking caused by concentration of heat stress generated at an upper end edge of the reference portion, thereby improving the productivity of the funnel.